In a multicolor electrophotographic (EP) reproduction apparatus comprising two or more image forming stations and an insulating paper transport web, the transfer of a toner image from an image carrier or toner image-bearing member (TIBM), i.e., a photoconductor (PC) or an intermediate transfer member (ITM), to a receiver, electrostatically or mechanically held to the paper transport web, is achieved by use of a constant current supplying device, i.e., a corona charger, brush or roller charger, which sprays or otherwise deposits a constant amount of charge, of opposite polarity from the toner, to the backside of the paper transport web, an example of such an apparatus is described in International published application W098/04961. As the receiver tacked to the paper transport web separates from the image carrier, in addition to the transfer charge located on the backside of the paper transport web, post-nip ionization charge of the same polarity as the toner is sprayed on the toned side of the receiver. For the case of positive wrap, i.e., wrap of the receiver about the image carrier, the magnitude of the post-nip ionization charge sprayed on the front side of the paper transport web defines the polar charge in the area of the receiver. The difference in the magnitudes of front-side and back-side charges defines the net charge in the area of the receiver. Polar charge due to post-nip ionization occurring during transfer at an upstream image forming station adversely impacts subsequent image transfers. FIG. 1 shows the reduction in the efficiency of toner transfer with increasing polar charge, since polar charge reduces the transfer field. As may be seen from FIG. 1, transfer efficiency decreases with increase in polar charge for images at maximum density, or halftones at 141 lines per inch with 40% coverage and even more dramatically for a halftone at 20% coverage. Net charge can cause image disruption during non-controlled air breakdown of the electric field produced by the net charge as the toned image on the receiver comes near a grounded plane.
To circumvent problems due to polar charge and net charge in a known prior art EP color machine wherein a receiver is transported by a paper transport web through the machine there are provided numerous corona chargers to condition the image and the receiver after transfer. Two conditioning chargers apply charge, opposite in polarity from the toner charge, to the front side of the toned receiver. Two conditioning chargers apply charge, opposite in polarity from the transfer charge, to the backside of the receiver. These chargers reduce both the polar charge and the net charge, insuring improved subsequent transfers. However, this image conditioning strategy is costly and produces excessive ozone.